1. Technical Field
This invention relates generally to luminescence techniques for radiation dosimetry, and, more specifically, to determining rapidly an unknown dose of radiation through optically stimulated luminescence.
2. Background
Luminescence techniques in radiation dosimetry are currently dominated by the method of thermoluminescence (TL) in which a sample (a thermoluminescence dosimeter, or TLD) is exposed to a certain dose of radiation and is then heated in the dark. At a certain temperature, or in a certain temperature range (either of which is dependent upon the material used and the detailed parameters of the heating procedure), luminescence is emitted from the material. The intensity (for example, the integrated light emission between two specified temperatures) is related, by calibration procedures, to the original absorbed dose of radiation. In this way, a method of radiation dosimetry is possible. This method of radiation dosimetry has been described in the literature, and has been in general usage, for approximately four decades. (See McKeever et al..sup.(1) for references to the early literature, and a summary of the state-of-the-art.)
As an alternative to thermal stimulation of the radiation-induced luminescence signal, optical stimulation is possible. Optically stimulated luminescence (OSL) was perhaps first suggested as a potential radiation dosimetry tool in 1955 by Antonov-Romanovskii.sup.(2) who suggested using infra-red light to stimulate luminescence from irradiated strontium sulfide. Later similar reports were presented by Braiunlich et al..sup.(3) and Sanborn and Beard.sup.(4). In each of these works the infra-red stimulated luminescence was continuously monitored during the light exposure--such a measurement mode is commonly referred to as "cw", or continuous wave, measurement--and the total luminescence detected was related to the initial absorbed dose. However, the sulfide materials used by these early investigators, and by more recent investigators.sup.(5), suffered from thermal instability and a high effective atomic number with an unacceptable energy dependence. Consequently, the use of IR-stimulated OSL from sulfides in dosimetry has not become established.
Several groups worked with wider band gap materials with acceptable effective atomic numbers and used light to transfer charge from deep traps to shallow traps, and then they monitored the phosphorescence decay from the irradiated materials. For example, in 1970 Rhyner and Miller.sup.(6) exposed samples of beryllium oxide to radiation, and subjected the irradiated materials to visible light for a specified period of time (up to 15 s). After a &gt;2 s delay following the light exposure the luminescence from the sample is monitored for a preset time (up to 120 s). An essentially identical procedure was described for measuring OSL from CaF.sub.2 :Mn by Bernhardt and Herforth.sup.(7) in 1974, and later by Henniger et al..sup.(8) in 1982. Berhardt and Herforth measured the intensity of the OSL emission 30 seconds after the end of the optical stimulation period (typically, 6 seconds long). Henniger et al., on the other hand, preferred to wait 10 s after the stimulation before measuring the integrated OSL between 10 s and 20 s. In each case a single light stimulation period was used. A very similar procedure is described by Pradhan and colleagues for monitoring OSL from CaSO.sub.4 :Dy.sup.(9-11). The latter authors used a 1 minute light stimulation and a 1 minute delay between the end of the excitation and the start of the measurement of the luminescence emissions.
An important aspect of the latter developments is that the delays between the end of the stimulation and the start of the measurement were purposely made long enough to exclude prompt OSL from the measurement and only to include that component of the OSL which is delayed by the action of trapping states (to be described later); indeed, this stipulation is specifically addressed in the papers by several of the authors. This method of OSL measurement, wherein there is a sufficient delay that prompt, or rapid, OSL emission is discriminated against by selection of a suitable time delay, is also known as "Delayed OSL" (or DOSL). To emphasize the fact that these authors are not using the prompt luminescence, but, rather, describe methods designed to monitor the delayed luminescence, the method is also called "optically stimulated phosphorescence"..sup.(7) Also note that in all of the above methods only one stimulation period is used in any one measurement. Furthermore, an important feature is that the length of the stimulation period, the length of the delay period (between stimulation and measurement), and the length of the measurement period are each significantly (orders of magnitude) longer than the lifetime of prompt OSL emission from the material.
Another similar technique is referred to as Cooled Optically Stimulated Luminescence (COSL)..sup.(12) Here the transfer of the charge from deep traps to shallow traps takes place at low temperatures (below room temperature) at which the transferred charge is stable in the shallow traps. The sample is then warmed to room temperature and during warming thermoluminescence emission is observed. The technique is, in fact, incorrectly described as an "optically stimulated" technique, and the older term for this process, phototransferred thermoluminescence (PTTL).sup.(13) is more accurate.
Several patents exist on the use of the above techniques for measuring absorbed radiation dose, including those by Gasiot et al..sup.(14) and Miller et al..sup.(15,16).
A development which emerged in the mid-1980s was the application of OSL in archaeological and geological dating. Here the goal is to determine the radiation dose absorbed by natural materials (archaeological or geological artifacts) while exposed to natural background radiation during burial over 100s-1000s of years. This application so was first described by Huntley et al..sup.(17) and involves the monitoring of the OSL emission simultaneous with the light stimulation. As with the IR-stimulated luminescence described above, the stimulating light is kept on the sample until the OSL signal has decayed to below the level of detection--i.e. the method of use is the cw-mode. This measurement mode is used within the dating community. Since the measurement of the luminescence is made simultaneously with the light stimulation, heavy filtering is required to discriminate between the stimulation light and the luminescence. Generally, these are of different wavelengths and one or other can be eliminated by the appropriate choice of optical filters. A second potential problem is the simultaneous stimulation of luminescence from non-radiation-induced defects within the sample which is also detected during measurement.
Another similar technology is called radiophotoluminescence (or RPL) in which a glass dosimeter is irradiated and then exposed to a fast (4 ns) laser pulse..sup.(18-21) The luminescence (RPL) following the end of the laser pulse is monitored. The radiation creates defects within the glass host and the laser light excites these into an excited energy state, from which relaxation back to the ground state results in the emission of luminescence. However, unlike the present invention, or any of the above-mentioned techniques, the laser light is not intended to empty electrons from radiation-induced trapping states, but merely to excite radiation-induced defects into higher, excited energy states from where relaxation to ground, or original, energy states can take place. After the laser stimulation the number of radiation-induced defects remains the same in the RPL method since transfer of electrons from one defect to another does not take place. A similar technique was reported earlier by Regulla.sup.(22), using LiF instead of phosphate glasses. Recent patents describe this technology using LiF..sup.(23,24)
Note should also be taken of laser heated thermoluminescence in which an irradiated sample is subjected to an intense infra-red beam from a CO.sub.2 laser. The sample (and/or the substrate to which the sample is attached) absorbs the IR light and is heated. The heating induces the emission of thermoluminescence. Several publications and patents exist describing this technology..sup.(25-30) The purpose of the laser light is to heat the sample, and in this way this technology differs substantially from the current invention.
References 1-30 listed in the following bibliography are incorporated by reference herein.
The current invention describes a method which measures the OSL emission in neither of the above two modes (i.e. neither DOSL, nor cw-OSL). The disadvantages of the above measurements modes are that only a small fraction of the OSL emission is delayed by the action of traps and thus DOSL is only capable of measuring this small component. In the cw-OSL method the heavy filtering which is required inevitably means that a substantial portion of the luminescence is also filtered and, therefore, is lost to the measurement. An additional disadvantage of both procedures is that the measurement process is extremely slow, taking place over several seconds, or tens of seconds (typically, up to 100 s).
This invention describes a method in which we discriminate against both the cw-OSL emission and DOSL emission, and instead monitor only that prompt OSL which emerges immediately after the cessation of the stimulation pulse (after a very short delay to allow the detection electronics to relax). The measurement mode to be described results in a significant enhancement of detected OSL signal and leads to a sensitive tool for the measurement of very small absorbed radiation doses. It is the intent of this invention to achieve fast measurements with high sensitivity over a wide dynamic range of radiation doses without encountering significant background signal interference or stimulation light leakage. The invented measurement mode is termed Pulsed-OSL (or POSL).